Key things to know about the British vote on whether to remain in the European Union

British voters face one of the most important decisions in a generation June 23: Do they want to remain in, or leave the 28-nation European Union, a partnership that began after World War II to generate economic cooperation and avoid war.

The latest polls on what has become known as Brexit show the two sides — Leave or Remain — neck and neck. In an unprecedented move, the deadline to register to vote was extended 48 hours last week after a surge of more than half a million people eager to sign up for the referendum caused a government website to crash.

For weeks, so-called battle buses for either side have been driving the length and breadth of the country trying to drum up support. Each campaign has thrust vocal, prominent representatives in front of microphones to champion its cause.

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No matter the outcome, Prime Minister David Cameron, who favors remaining with the EU, will have a fight on his hands within his Conservative Party, which has become deeply, and very publicly, divided on the issue.

Here is a look at the EU referendum and why it matters:

What is the European Union?

The EU has its own parliament, commission and courts. It operates under a single market that enables the free movement of people, goods and services and imposes standardized laws. Passport controls were abolished within the so-called Schengen Area, but Britain is not part of it. Nineteen of the 28 EU countries are part of the Eurozone, meaning that they use the euro as their currency. Britain voted against joining the Eurozone and still uses the pound sterling.

Why is a referendum being held?

This referendum was a key part of Cameron’s election manifesto. His Conservative Party has long been divided over the nation’s relationship with Europe and he hoped that a vote on EU membership would quell the “Euroskeptics” within his party. He also hoped it would counter the rising popularity of the UK Independence Party led by Nigel Farage, which campaigns on an anti-migration platform.

Before setting a date for the referendum, Cameron spent weeks meeting EU leaders to renegotiate the terms of Britain’s membership in the EU. He argues that the reforms he secured give Britain a “special status” within the EU and address some of the issues that people dislike about the EU, including high levels of immigration and less sovereignty.

What are the key issues?

Immigration: The Leave campaign says Britain has lost control of its borders, as there is currently no way to stop EU citizens from coming to Britain to work, study or live. Net migration to Britain was 333,000 last year, according to the Office of National Statistics, and 184,000 of those people arrived from within the EU. The Remain camp says EU migrants fuel economic growth. Many Eastern Europeans work as builders or cleaners in Britain for example, jobs that many British citizens don’t want to do. Remain campaigners also warn that a vote to leave would see the British border, which is across the English Channel in France, moved to Dover on British soil.

Security: Those who want to leave the EU say Britain is more at risk of a terrorist attack if it remains part of the EU because it cannot control who enters the country as adequately as if it were independent. This argument has been challenged by several senior military figures who argue that being in the EU and able to collaborate on intelligence makes Britain safer.

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Trade: Britain is part of the single EU market, so it faces no trade tariffs on imports and exports. The EU also has trade deals with other world powers, such as the U.S. Britain could lose those benefits if it were to leave the EU. President Obama said a Brexit would see Britain put to the “back of the queue” when it came to trade talks. Those who want Britain to leave, however, argue that it could negotiate its own trade agreements and would not be burdened by restrictive EU law.

Sovereignty: Leave campaigners want Britain to regain its sovereignty as an independent nation that controls its own laws. Those voting to remain say the future is about countries collaborating to flourish. On Thursday, former British Prime Minister Tony Blair said: “The future Is not about countries breaking apart their alliances. The future is about countries coming together.”

What happens if Britain votes to leave?

If Britain voted to leave the EU, it would have to renegotiate its relationship with the remaining EU members and the world. The Leave camp says this would ultimately place Britain in a better financial position, especially because Britain contributes billions of pounds to the EU each year that could be used to boost its economy.

The Remain camp, as well as leading economists, say the benefits of being part of the EU far outweigh any problems and leaving could negatively affect jobs, living standards, housing prices and long-term economic stability.

“This is not something to be given up easily, is it something that is actually part of our lives that we take for granted,” former Prime Minister John Major said Thursday during a joint news conference with Blair. “So don’t take this vote for granted. Make sure that you do actually come out, and vote to stay.”

If the Leave camp wins, there are fears that Scotland, which saw a vote on independence from Britain defeated in 2014, would demand a second referendum. Northern Ireland would also have to introduce a land border with Ireland, which is an EU member, a move that some say could threaten the peace process there.

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Boyle is a special correspondent.